Hydrogen sulphide (H2S) is a colourless gas, typically arising from natural sources, such as hot springs, volcanic gases, and natural gas, and from anthropogenic processes such as coal gasification, wastewater treatment, and petrochemical refining.
H2S is a contaminant in fuel process streams that can damage equipment even at low concentrations. H2S at concentrations as low as 3 ppm can result in severe pipeline corrosion (Baird et al., 1992). Trace amounts of H2S in natural gas may poison nickel or alumina catalysts used in steam reforming processes. H2S negatively impacts ceramic membranes used in syngas separations. The anodic platinum catalysts of proton exchange membrane (PEM) fuel cells are susceptible to H2S at concentrations of 0.1 to 1 ppm (Shekhawat et al., 2011).
Accordingly, certain industrial applications need to “polish” gas streams so that the H2S concentration is reduced to sub-ppm levels. Conventional methods for H2S sequestration include the Claus process, adsorption in liquids (e.g., alkaloamines, ammonia solutions, and alkaline salt solutions), and adsorption by activated carbon, solid supported amines, or metal oxides (Stirling, 2000; Quanmin et al., 2012). Of the various H2S removal technologies available, only reactive metal oxides have the capability and capacity to remove significant amounts of H2S to concentrations as low as several parts per billion (ppb) for low-temperature catalytic applications, or up to tens of ppms for combustion-temperature applications.
Considerable attention has been paid to using and enhancing the performance of metal oxide adsorbents at high temperatures. Oxides of iron, zinc, molybdenum, manganese, vanadium, calcium, strontium, barium, cobalt, copper and tungsten have all been identified as possible candidates for desulphurization at high temperatures in excess of 360° C. (Westmoreland et al., 1976). Only copper oxide has the ability to effectively and efficiently remove H2S at both low and high operating temperatures.
The adsorptive performance of metal oxides may be enhanced by doping with other metals. For example, zinc ferrite may be used for the desulphurization of coal gases at 538° C. (Ayala et al., 1991). A study of oxides of copper mixed with chromium, cerium, aluminum, magnesium, manganese, titanium and iron found that CuO—Cr2O3 and Cu—CeO2 were the most efficient adsorbents at 650° C. (Abbasian et al., 1992).
The adsorptive performance of metal oxides may also be enhanced by loading them onto supports such as Al2O3, TiO2, SiO2, and zeolites to increase their structural stability and reactive surface area. A comparison of the adsorption capacities of manganese, iron, copper, cobalt, cerium and zinc supported on γ-Al2O3 for H2S removal in syngas between 500° C. and 700° C. found that 100% utilization was achieved using copper and manganese (Ko et al., 2005). A study of copper, molybdenum and manganese supported on SP-115 zeolite for desulphurization purposes noted an increase in the mechanical strength of the adsorbents (Gasper-Galvin et al., 1998). A comparative study demonstrated that the breakthrough capacities at 600° C. of copper supported on SiO2 (15 wt. % copper) and copper supported on natural zeolite (major phases consisting of mordenite and clinoptilolite) (20 wt. % copper) were almost the same as that for pure copper oxide (Kyotani et al., 1989). Although copper exchanged zeolite molecular sieves result in virtually complete metal utilization due to the atomic dispersion of the metal and the high surface area of the support, the use of molecular sieves is not practical in all circumstances, primarily due to fouling of the micropores and restricted thermal stabilities.
Relatively less attention has been paid to the use of metal oxides as H2S adsorbents at low temperatures, as is required in catalytic applications such as proton exchange membrane (PEM) fuel cells. At low temperatures, H2S can only react with the first monolayer of metal oxides (Baird et al.). The material at the center of the particle, inaccessible due to the metal sulphide layer on the surface remains unreacted, thereby reducing the utilization of the metal in the adsorbent and decreases adsorbent efficiency. A comparative study of oxides of silver, copper, zinc, cobalt, nickel, calcium, manganese, and tin exposed to 1 ppm using 10 ppm H2S in nitrogen at room temperature demonstrated that hydrous copper oxide has the highest H2S uptake capacity (Xue et al., 2003). A comparative study of H2S uptake capacity at room temperature found that among ZnO/SiO2 doped with eight different transition metals, ZnO/SiO2 doped with copper had the highest capacity, which was twice that of ZnO/SiO2 (Yang et al., 2010). It has also been demonstrated that copper and cobalt doping agents enhance the sulphur removal capacity of zinc oxide at room temperature (Baird et al., 1992). Commercially, aggregates of ZnO are used for deep H2S polishing applications where the feed stream is near ambient temperature. Although this absorbent is sufficiently cost-effective for use in sacrificial guard beds, the material performance is poor under these conditions. An alternative absorbent is R3-11G (BASF Chemical Company), a composite copper oxide material composed of high surface area, engineered nano-scale copper oxide particles. Although this absorbent has no microporosity and greatly improved metal utilization, the technology involved in engineering the nano-scale particles adds substantial cost.
Notwithstanding advances in H2S absorbents, there remains a need in the art for an adsorbent that has high adsorption capacity over a range of temperatures, particularly at lower temperatures.